Most metal alloys forms crystals with systematic atom arrangement upon solidification from the liquid phase. However, if the cooling rate is large enough over the critical value and thereby, nuclear formation of the crystal phase is suppressed, unsystematic atom structure in the liquid phase can be maintained. An alloy having such structure is called an amorphous alloy, and an alloy particularly containing metal atoms is called metallic glass alloy.
Since the first report of a metallic glass phase in an Au—Si alloy in 1960, many kinds of amorphous alloy have been proposed and used. However, most amorphous alloys are only prepared in the form of ribbons having a thickness of about 80 μm or less, micro wires having a diameter of about 150 μm or less or powder having a particle size of several hundreds gm or less using the rapid quenching method with a high cooling rate of 104 to 106 K/s.
Therefore, since there is a limit in shape and size in the preparation of the amorphous alloys by the rapid quenching method, the amorphous alloys cannot be used for industrial application as a structural material but a part of them can be used for industrial application as a functional material such as magnetic materials.
Thus, in order to meet the need for application as a advanced-functional/structural metal material, it is desired to have an alloy composition which has excellent glass forming ability and can form amorphous phase at a low critical cooling rate and can be cast with a bulk amorphous material.
By U.S. Pat. Nos. 5,288,344 and 5,735,975, it is known that amorphous alloys having a critical cooling rate of about several K/s and a very wide supercooled liquid region can be formed in a predetermined shape for use as a structural material. The Zr—Ti—Cu—Ni—Be alloy and the Zr—Ti—Al—Ni—Cu alloy are already used as bulk amorphous products. In addition, novel bulk amorphous alloys are developed from various alloys such as nickel-based, titanium-based or copper-based alloys and evaluated to have useful and characteristic properties such as excellent corrosion resistance and strength. For example, according to Materials Transactions (JIM, Vol. 40 (10), pp. 1130-1136), a bulk amorphous alloy having a maximum diameter of 1 mm is prepared from Ni—Nb—Cr—Mo—P—B by copper mold casting. This bulk amorphous alloy has a relatively wide supercooled liquid region.
Also, U.S. Pat. No. 6,325,868 discloses a bulk amorphous alloy having a maximum diameter of 3 mm based on Ni—Zr—Ti—Si—Sn by copper mold casting. This bulk amorphous alloy also has a relatively wide supercooled liquid region.
Further, according to Applied Physics letters (Vol. 82, No. 7, pp. 1030-1032), a bulk amorphous alloy having a maximum diameter of 3 mm is prepared from Ni—Nb—Sn by copper mold casting.
Meanwhile, Fe-based amorphous alloys have been used usually as a magnetic material for several tens years. Recently, alloys that can be cast to a size of several mm or more have been developed and actively studied for their application as advanced-functional structural material. For example, Professor Poon et al. in the University of Virginia have reported that an amorphous rod having a size of 12 mm can be prepared from an alloy based on Fe—Cr—Mo—(Y, Ln)—C—B (Journal of Materials Research Vol. 19 No. 5, pp. 1320-1323).
However, those bulk amorphous alloys developed in the prior art have problems in terms of industrial application, as follows.
Firstly, since the glass forming ability of the alloys is greatly affected by an impurity content in raw materials of the alloys, expensive high purity materials should be used and the material processing upon dissolution and casting should be precisely performed under a special atmosphere such as vacuum or Ar (argon) gas atmosphere.
Secondly, since most alloys contain rare metals such as Er (erbium), Y (yttrium) and the like or a large amount of expensive atoms such as Mo (molybdenum) and Cr (chromium), there is a problems related with increase in the production cost including unit cost of raw materials and additional cost caused by dissolution and use of a special furnace.
Thirdly, the conventional bulk amorphous alloys have much higher viscosities in the liquid phase, as compared to general metals and thus, poor castability, which presents a limit in the casting and product design. Therefore, though the conventional bulk amorphous alloys have very unique and beneficial properties, they can be prepared only experimentally and have problems related with the production cost and difficulties in application of the process for mass-production using existing equipments.
Therefore, it is desired to have a Fe-based amorphous alloy composition which has excellent castability and can be prepared by economical raw materials and process and so that the properties of bulk amorphous alloys can be applied in practical industries.